fbdev: amifb: Replace zero-length arrays with DECLARE_FLEX_ARRAY() helper

Replacing zero-length arrays with C99 flexible-array members
because they are deprecated. Use the new DECLARE_FLEX_ARRAY()
auxiliary macro instead of defining a zero-length array.

This fixes warnings such as:
./drivers/video/fbdev/amifb.c:690:6-10: WARNING use flexible-array member instead

Signed-off-by: Atul Raut <rauji.raut@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>